There are a handful of spells listed in the Player’s Handbook that are actually more likely to be used by the DM than any self-respecting hero, and few crop as commonly as the good old Arcane Lock.
In many ways it does exactly what you would expect – sealing up a door, window or chest with mystical force. The trappings of the fantasy genre mean that it can be temporarily turned off by a word or a phrase, or simply be instructed to let a specific group of people pass without issue.
Of course, it’s entirely possible to run up against the spell without really noticing it. Many DMs handwave away the technical details of the non-combat spells wielded by NPCs (How does that extraplanar portal work? Magic, that’s how!), but if we’re being strict with the rules then every time you “Speak Friend and Enter” or gain entrance to the Wizard’s sanctum by guessing his favourite type of candy you’re actually interacting with an Arcane Lock.
Playing by the rules does introduce a couple of issues, however. For one thing, it specifies that the spell can be suppressed by Knock and that it doesn’t make the door/chest/whatever impossible to pick or break down, but rather increases the DC by 10. If the door is half-decent this will usually be enough to propel it into the realms of the virtually impossible, but a handful of lucky rolls or a willingness to throw enough time into the task will allow a party to skip the riddle or task and just force the door.
This isn’t necessarily a bad thing, of course. Sometimes people are just going to stumble over a puzzle and the adventure needs to progress somehow, plus it makes it clear to the players that they can’t make their own lairs impregnable with the same magic.
This kind of fortification is the most common way for players to use Arcane Lock, but there are some fringe cases when it can be useful. If they’re particularly paranoid about being attacked on the road or having their valuables pilfered it can keep out all but the most talented of attackers – though the 25gp cost per casting can make it a little expensive for low-level adventurers to use regularly.
While the 10 minute casting time means it’s very rare for it to be used in any kind of hostile situation, some careful planning can secure an escape route that pursuing enemies will find incredibly hard to breach.
Ultimately, Arcane Lock is a spell that is much more functional than it is exciting. If nothing else, it gives you a chance to turn the tables and get the NPCs trying to guess your elaborate riddle for a change.
Pretty much exactly what you want from this kind of spell. It does exactly what you would want it to do, with the long cast time and non-trivial cost keeping it thoroughly balanced. Not something you’d want to prepare, but worth keeping in the spellbook for the occasional ritual casting.
Casting Time: 1 action
Components: V, S, M (gold dust worth at least 25 gp, which the spell consumes)
Duration: Until dispelled
You touch a closed door, window, gate, chest, or other entryway, and it becomes locked for the duration. You and the creatures you designate when you cast this spell can open the object normally. You can also set a password that, when spoken within 5 feet of the object, suppresses this spell for 1 minute. Otherwise, it is impassable until it is broken or the spell is dispelled or suppressed. Casting knock on the object suppresses arcane lock for 10 minutes.
While affected by this spell, the object is more difficult to break or force open; the DC to break it or pick any locks on it increases by 10.